1. Field of the Invention
The present invention relates to an optical disc reproduction device using an optical disc such as CD (Compact Disc) or DVD (Digital Versatile Disc), and a tracking control method applied to the optical disc reproduction device.
2. Description of the Related Art
A conventional optical disc reproduction device for reproducing information, such as images, sounds and characters, recorded on an optical disc such as CD (Compact Disc), LD (Laser Disc), DVD (Digital Versatile Disc), DVD-ROM (DVD-Read Only Memory) or MD (Mini Disc) is configured such that laser light is irradiated onto a disc surface of the optical disc, and a photodetector in an optical pickup converts the light reflected from the disc surface into electric signals. Based on the electric signals, the optical disc reproduction device reads the information recorded on the optical disc, and corrects focus deviations of the light irradiated onto the disc surface. For example, a photodetector, which is used for a focusing servo mechanism based on the astigmatism method and which has a light receiving portion divided into four regions, outputs four detection signals corresponding to the respective light receiving regions. Based on the detection signals, the optical disc reproduction device generates various signals such as RF (Radio Frequency) signals, focus error signals and tracking error signals.
Meanwhile, an optical disc reproduction device is designed to suppress, as much as possible, jitters such as: jitter due to tilt of e.g. a disc tray; jitter influenced by warpage of an optical disc; jitter due to waveform interference caused by light diffraction when reading an optical disc, and/or due to aberrations of a reproduction optical system, and/or due to influence of noises, and/or due to lowered level of detected signals; and jitter of an electric circuit system.
A conventional optical disc reproduction device automatically performs focus adjustment and tracking adjustment when it starts reading an optical disc. For the focus adjustment, the optical disc reproduction device performs adjustment such that a light beam or laser light from an optical pickup is focused on a recording surface of the optical disc at an optimum point for jitter where an RF signal becomes maximum. For the tracking adjustment, the optical disc reproduction device performs adjustment such that the laser light accurately traces a track on the recording surface of the optical disc.
For tracking control, the conventional optical disc reproduction device adjusts a tracking balance signal value (hereafter often referred to simply as “TBAL signal value”) such that a tracking error balance value (hereafter often referred to simply as “TE Balance value”) becomes 0% (zero percent), unconditionally assuming that a TBAL signal value to make the TE balance value 0% coincides with a TBAL signal value to minimize the jitter. However, this assumption may not always be correct e.g. as shown in FIG. 4. FIG. 4 is a graph showing an example of TBAL-Jitter characteristics of a DVD-R (DVD-Rewritable) where the left vertical axis represents jitter value and the right vertical axis represents TE Balance value, while the horizontal axis represents TBAL signal value, for explaining a tracking control problem in the conventional optical disc reproduction device. In FIG. 4, curves A and B show jitter at an inner circumference (inner jitter) and jitter at an outer circumference (outer jitter) of the disc, respectively, while broken-line C and line D show TE Balance value and approximated line of the TE Balance value (TE Balance line). As indicated in FIG. 4, the jitter may not always be minimum at the TE Balance value of 0%, e.g. due to warpage or flaws of the optical disc. Accordingly, the conventional optical disc reproduction device has a problem that it cannot perform balanced tracking control with a good balance between tracking performance and jitter performance.